FDC2214: Very large temperature influence

I'm using the FDC2214 to indicate soil moisture in soils, this works due to different capacitance in dry soil vs wet soil. The FDC2214 chip is connected with three sensor areas connected to 3 out of 4 channels of the chip.
 am noticing a very large temperature influence on the measurements on all three channels. I'm not sure how to limit or correct for this temperature influence. I'm using the internal oscillator (-13 ppm/C) with the following settings for all channels:

Hopefully any of you can give an idea where to look for this temperature influence and maybe how to correct for this since the temperature near the FDC2214 is also measured. My best guest now is that the internal oscillator is deviating too much